Little is known about the relative influence of genetic and environmental factors in alcohol and drug abuse/dependence as characteristics of individuals. As population characteristics (i.e. heritability), estimates across multiple studies suggest that approximately one half of the liability for these disorders in males and one fourth in females is due to genes. While such population level measures are useful in establishing the relative importance of genetic factors, they are unable to help dissect the confounded relationship of genes and environment. To tease apart the etiologic influence of genes, the environment, and their interaction it is necessary to be able to assess the relative contribution of these factors in individual cases of dependence syndromes. Measures of genetic and environmental influences in individuals will allow for investigation of clinical, and pre-clinical features associated with each etiologic factor, the development of genetic and environment phenotypes, and identification of alcohol and/or drug dependent samples for whom genetic influences appear particularly strong which would increase the efficiency of linkage and association studies. Comparisons of identical twins' (MZ) and fraternal twins' (DZ) probandwise concordance rates provides a method by which abuse/dependence symptoms can be assessed for the presence of genetic loading for each symptom. Symptoms for which MZ/DZ differences are found can be combined to form a genetic influence scale. Those symptoms for which no such statistically significant difference is found constitute items in an environmental influence scale. Complete Diagnostic Interview Schedule version III (DIS III) interviews, other clinical features and family history information are available on 169 same sex twin pairs.